1. Field of the Invention
The present invention relates to a storage apparatus which reads and writes information from and into a medium rotated by a spindle motor, a spindle motor control method and a program. More particularly, the invention relates to a storage apparatus which controls a loop gain to an optimum value for servo-controlling a spindle motor so as to ensure rotation of a medium at a constant speed, a spindle motor control method and a program.
2. Description of the Related Art
In a storage apparatus such as a magnetic disk apparatus, a brushless DC motor as conventionally been used as a spindle motor rotating a recording medium at a constant speed, and it has been the conventional practice to set a target rotational speed by a spindle motor control unit, detect revolutions of the brushless DC motor from the counter electromotive force of the stator coil, and driving current of each phase to the spindle motor is servo-controlled so as to eliminate a difference from a target revolutions by feeding back the result of detection.
The duty ratio of driving current fed to each phase of the motor is controlled so as to eliminate the difference in speed, and noise is reduced by expressing the peak change in switching current waveform caused by the control of duty ratio in a sine wave.
More recently, the field of application of the magnetic disk apparatus has expanded, not limited to desktop type and note type personal computers, but also to car navigation systems and AV devices. For the purpose of enhancing the mass-producibility and reducing cost, apparatuses in which only the number of media is different and all other components including an enclosure, disk enclosure, circuit board and firmware are made common are now being manufactured.
Usually, when designing spindle motor control units used for apparatuses of different numbers of media, the fact should be taken into account that the difference in the number of media leads to a difference in inertia of the spindle motors. It is therefore the common practice to determine a loop gain of the servo loop so that control properties of the spindle motor is not affected by variations of this inertia, and to make it possible to accomplish common control irrespective of the number of media.
There is available a related patent document JP 9-198820.
In such a conventional spindle motor control unit for coping with apparatuses of different numbers of media, a servo gain is determined so as to cover variations of inertia resulting from the difference in the number of media. However, as compared with a case of optimization of the loop gain of the spindle motor control unit with a fixed number of media, stability against disturbance such as temperature and source voltage is not perfect.
For example, when there is a change in environmental temperature of the apparatus, the magnet intensity in the spindle motor changes under the effect of temperature, and furthermore, particularly when using a fluid bearing, variation of the oil viscosity leads to a change in the loop gain of the servo loop. As a result, a problem is encountered in that a desired control property cannot be maintained for the spindle motor under a temperature condition within a range of service temperatures, for example, from 0° C. to 60° C.
When a change in source voltage occurs, PWM control of motor current may cause a change in loop gain under the effect of voltage variation. PWM control of motor current causes a change in duty ratio so as to eliminate the difference in speed. Since a lower source voltage corresponds to a decrease in current and hence in speed, leading to a larger difference in speed, thus causing an increase in duty ratio. This change in duty ratio causes a change in loop gain and sometimes prevents a desired control property from being kept.
In an actual apparatus, changes in temperature and changes in source voltage compositely occur. It is therefore difficult to maintain the spindle motor with desired control properties in all possible environments relative to changes in the number of media.